The present invention relates to solid oxide fuel cells (SOFCs), particularly to SOFCs utilizing cobalt iron based electrodes, and more particularly to a method for fabricating SOFCs wherein a buffer layer of doped-ceria is deposited by a colloidal spray deposition technique intermediate the zirconia electrolyte and the cobalt iron manganese based electrode, whereby the power density of the SOFCs is increased 2-3 times over conventionally known cells.
(Solid Oxide Fuel Cells) SOFCs are solid state electrochemical devices that convert chemical fuels directly to electricity. Because no combustion is involved, SOFCs are not limited by the Carnot cycle. As a result, SOFCs can have efficiency much higher than conventional power generation devices. The use of SOFCs would result in fuel economy and lower carbon emissions. For these reasons, SOFCs are currently of high interest for clean and efficient electricity generation for stationary and transportation applications.
However, despite many successful demonstrations by Siemens Westinghouse, the SOFC commercialization is still not envisioned for a close future because of the excessive high cost. Such a high cost comes from the inherent low power density of the Westinghouse tubular design (300 mW/cm2 at 1000xc2x0 C.) and particularly from the manufacturing cost due to the expensive processing techniques. An alternative to the Westinghouse tubular fuel cell is the planar design that has potentially higher power density and can be operated at lower temperatures, 800xc2x0 C. or lower. The operation at intermediate temperatures makes possible the use of cheaper materials such as the metal interconnect and while putting less constraints on the materials and the gas manifolding system. However, planar fuel cell technology is less mature and a number of issues must be resolved before possible commercialization. In particular, although certain alloys can be used at 800xc2x0 C., they still tend to oxidize severely after several hundred hours of operation. Therefore, there is strong interest to further decrease this operating temperature to below 700xc2x0 C. Unfortunately, most of current planar SOFCs loss rapidly performance when temperature decreases. For example, the Honeywell (ex Allied Signal) fuel cell power density drops from 650 mW/cm2 at 800xc2x0 C. to 350 mW/cm2 at 700xc2x0 C., that of the European ECN fuel cell drops from 610 mW/cm2 at 800xc2x0 C. to 270 mW/cm2. Thus, current SOFCs are not suitable for operation at temperature below 700xc2x0 C. It is noted that very high power density SOFCs, up to 2W/cm2 at 800xc2x0 C. and 1 W/cm2 at 700xc2x0 C., have been reported by Lawrence Berkeley National Lab and the University of Utah. However, the measurement conditions are unclear. Recently, we have shown that certain testing configurations can yield largely over estimated power density. When single cells are tested in the asymmetric configuration, i.e. where one of the electrodes is significantly larger than the other one and the power density is then normalized to the smaller electrode area, such artificially increased the power density by the favorable normalization effect. The asymmetric cell configuration does not correspond to the actual fuel cell stack operating condition. Therefore, all comparisons should not take into account data obtained using asymmetric configuration.
It is well known that (La,Sr)(Co,Fe)O(LSCF)is a much better cathode material than the conventional (La,Sr)MnO electrode. Unfortunately, the cobalt iron based electrode tends to react with the zirconia electrolyte, causing rapid long-term degradation. A buffer layer of doped-ceria has been proposed to avoid the direct contact of the LSCF electrode with the zirconia electrolyte. Although the concept is interesting, the reduction to practice has not been successful because of the lack of an adequate technique to deposit the buffer coating. For instance, doped-ceria has been deposited on a zirconia electrolyte using sputtering or conventional screen-printing techniques. Cracking of the buffer layer has been observed because of the difference in thermal expansion coefficients between the two layers. No significant performance improvement has ever been reported.
The present invention provides a solution to the above-mentioned problem regarding the formation of a buffer layer between an LSCF electrode and a zirconia electrode. This is accomplished by depositing a doped-ceria buffer layer using a colloidal spray deposition (CSD) technique such as described and claims in U.S. Pat. No. 6,358,567, filed Apr. 16, 1999, entitled xe2x80x9cColloidal Spray Method for Low Cost Thin Coating Depositionxe2x80x9d, whereby the doped-ceria buffer layer is deposited on the zirconia layer without cracking, and then the LSCF electrode is subsequently deposited on top of the buffer layer using the same CSD technique. Thus, the power density of the SOFCs formed by the method of the present invention is increased by 2-3 times over that of the conventional fabricated SOFCs.
It is an object of the present invention to provide high power density solid oxide fuel cells.
A further object of the invention is to provide a method for depositing a cobalt iron based electrode on a zirconia electrolyte.
Another object of the invention is to provide a high power solid oxide fuel cell utilizes a zirconia electrolyte, a buffer layer, and a cobalt iron electrode.
Another object of the invention is to provide a method for producing solid oxide fuel cells which includes depositing by a colloidal spray deposition technique, a zirconia electrolyte, a doped ceria buffer layer intermediate, and a cobalt iron based electrode, whereby the power density of the cell is increased 2-3 times over conventional cells.
Other objects and advantages of the present invention will become apparent from the following description and accompany drawing. The present invention involves high power density solid oxide fuel cell (SOFC) and method of fabrication. The SOFC fabricated by this invention utilizes a buffer layer between the electrode and the electrolyte formed by a colloidal spray deposition (CSD) technique wherein doped-ceria, for example, as the buffer material can be deposited on a zirconia electrolyte, for example, without cracking, thus solving the above-referenced problems relative to the use of doped-ceria buffer layers for a SOFC fabricated from an (La,Sr)(Co,Fe)O (LSCF) electrode and a zirconia electrolyte. The doping element for ceria can be any element of the lanthanides, but preferably gadolinium or yttrium. By this invention composite cathodes may be produced by a mixture of doped-ceria with LSCF instead of the single component LSCF electrode, resulting in higher performance. SOFC produced by the method of this invention have shown a peak power density of 1400 mW/cm2 at 800xc2x0 C. using hydrogen fuel which is a factor of about two higher than the above-reference SOFCs. Also, at 700xc2x0 C., the peak power density was 900 mW/cm2. The CSD technique used to produce at least the doped-ceria buffer layer may be carried out by the method of above references application Ser. No. 09/293,446.